Kurzgesagt – In a Nutshell
Kurzgesagt – In a Nutshell
Embedded Files
Sources – Cosmic Crisis
Sources – Cosmic Crisis
We thank the following experts for their comments:
Dr. Eleonora Di Valentino
University of SheffieldDr. Miguel Zumalacárregui
Albert Einstein InstituteDr. Matthew Caplan
Illinois State University
—Thanks to amazing new technology, our understanding of the universe is moving faster than it has in years.
New instruments and technologies have allowed cosmologists to gather loads of new data.
Examples of these technologies are more powerful telescopes,
#Webb Space Telescope: “The Observatory: The Next Generation” (retrieved 2025)
https://webbtelescope.org/science/the-observatory
new ways of collecting and processing light to create vast maps of the universe,
#Sloan Digital Sky Survey: “The Sloan Digital Sky Survey: Mapping the Universe” (retrieved 2025)
#DESI: “mapping the universe” (retrieved 2025)
https://www.desi.lbl.gov/focal-plane-system/
or even instruments that allow us to detect gravitational waves
#NSF LIGO: “About” (retrieved 2025)
—For decades, we’ve had a beautiful theory of the cosmos. One that explained how the universe began, what it’s made of, and how it’s supposed to behave.
The “beautiful theory of the cosmos” refers to the ΛCDM model, considered the “standard model” of cosmology.
See the following link for an introduction to the ΛCDM model:
#Grayson, Skylar et al. (2025): "Guide to ΛCDM”, Astrobites
https://astrobites.org/2025/01/06/lambda_cdm/
The ΛCDM model together with inflation explains the evolution of the universe from its earliest instants
#NASA Goddard Space Flight Center: “ΛCDM Model of Cosmology” (retrieved 2025)
https://lambda.gsfc.nasa.gov/education/graphic_history/univ_evol.html
what it is made of
#ESA Euclid (2025): “Mission science”
https://sci.esa.int/web/euclid/-/mission-science
and how it will behave in the future
#NASA WMAP's Universe (2024): “Universe 101: What is the Ultimate Fate of the Universe?”
https://wmap.gsfc.nasa.gov/universe/uni_fate.html
—It matched our observations amazingly well and made us feel like we’d almost solved the cosmic code.
Observations like the 2013 Plank results were in overall excellent agreement with the predictions of the ΛCDM cosmology
#Planck Collaboration (2014): “Planck 2013 results. XVI. Cosmological parameters”, Astronomy & Astrophysics, vol. 571, A16
https://www.aanda.org/articles/aa/abs/2014/11/aa21591-13/aa21591-13.html
Quote: “This paper presents the first cosmological results based on Planck measurements of the cosmic microwave background (CMB) temperature and lensing-potential power spectra. We find that the Planck spectra at high multipoles (ℓ ≳ 40) are extremely well described by the standard spatially-flat six-parameter ΛCDM cosmology with a power-law spectrum of adiabatic scalar perturbations.
[...]
The most important conclusion from this paper is the excellent agreement between the Planck temperature power spectrum at high multipoles with the predictions of the base ΛCDM model. The base ΛCDM model also provides a good match to the Planck power spectrum of the lensing potential, Clφφ, and to the TE and EE power spectra at high multipoles.”
And were interpreted as a “confirmation” of ΛCDM cosmology, even if they also showed the first signs of what would become important tensions.
#ESA (2018): “From an almost perfect Universe to the best of both worlds”
Quote: “When the image was revealed, the data had confirmed the model. The fit to our expectations was too good to draw any other conclusion: Planck had showed us an ‘almost perfect Universe’. Why almost perfect? Because a few anomalies remained, and these would be the focus of future research.
Now, five years later, the Planck consortium has made their final data release, known as the legacy data release. The message remains the same, and is even stronger.
“This is the most important legacy of Planck,” says Jan Tauber, ESA’s Planck Project Scientist. “So far the standard model of cosmology has survived all the tests, and Planck has made the measurements that show it.””
—At first they looked like silly mistakes, noise that would go away with more data. But as new data came in, the opposite happened. Some cracks got larger, new ones emerged, and our once perfect picture of the cosmos began to look less and less… perfect.
#CNRS News (2023): “Cosmology in turmoil”
https://news.cnrs.fr/articles/cosmology-in-turmoil
Quote: “At first glance, the discrepancy with the result from the analysis of the cosmic microwave background seems minor. However, when uncertainties are taken into account, the two estimates are incompatible. “Until a few years ago, there was still some doubt about the reality of this anomaly. However, measurements of H0 have now become so precise that the probability that this discrepancy is due to chance is less than one in a million. That’s very exciting, because it could mean that our model of cosmology is incomplete and that we need to consider the possibility of new physics,” says Vivian Poulin-Détolle, of the Montpellier Universe and Particles Laboratory (LUPM).”
#Abdalla, Elcio et al. (2022): “Cosmology intertwined: A review of the particle physics, astrophysics, and cosmology associated with the cosmological tensions and anomalies”, Journal of High Energy Astrophysics, vol. 34, 49-211
https://www.sciencedirect.com/science/article/pii/S2214404822000179?via%3Dihub
https://arxiv.org/abs/2203.06142
Quote: “The 2018 legacy release from the Planck satellite [23] of the Cosmic Microwave Background (CMB) anisotropies, together with the latest Atacama Cosmology Telescope (ACT-DR4) [26] and South Pole Telescope (SPT-3G) [106] measurements, have provided a confirmation of the standard ΛCDM cosmological model. However, the improvement of the methods and the reduction of the uncertainties on the estimated cosmological parameters has seen the emergence of statistically significant tensions in the measurement of various quantities between the CMB data and late time cosmological model independent probes. While some proportion of these discrepancies may eventually be due to the systematic errors in the experiments, their magnitude and persistence across probes strongly hint at a possible failure in the standard cosmological scenario and the necessity for new physics.”
—Two centuries ago, astronomers noticed that Uranus’ orbit didn’t quite follow the laws of gravity. But instead of throwing those laws away, they proposed that a “dark planet” was tugging on Uranus from afar. Shortly after, Neptune was discovered – exactly where the math said it would be.
#Rodríguez-Moris, Gabriel; Docobo, José A. (2024): “The Discovery of Neptune Revisited” (Preprint)
https://arxiv.org/abs/2405.06310
Quote: “Following the discovery of Uranus, astronomers tried to predict its future positions using perturbation theory applied to the exactly integrable Newtonian 2-body problem in Celestial Mechanics. However, these were rapidly found to differ from the actual observed positions. These mismatches led some astronomers of the time, such as the director of the Paris Observatory Alexis Bouvard, to propose, among other hypotheses, the existence of a body farther away than Uranus whose perturbations on its orbit were responsible for them. Within this framework, John Couch Adams and Urbain Le Verrier reached, independently but using essentially the same perturbation theory techniques, a predicted position on the celestial sphere for the supposed perturber , which was found to be very close to the actual position spotted in September of 1846 by Johann Gottfried Galle and Heinrich Louis d’Arrest at the Berlin Observatory, a truly remarkable achievement for the epoch. This led to the discovery of planet Neptune, the eighth planet of the Solar System.”
—But then came Mercury. Its orbit also didn’t make sense, so scientists tried the same trick.
#Pollock, Chris (2003): “Mercury’s Perihelion”
https://www.math.toronto.edu/~colliand/426_03/Papers03/C_Pollock.pdf
Quote: “In 1859, [Le Verriere] began examining observations of Mercury’s motion in detail, relying mainly on 14 very accurate solar transit times, recorded between 1697 and 1848. He concluded that the ellipse of Mercury’s orbit was precessing slowly, which was expected, since the other planets of the solar system were expected to Mercury’s perihelion to precess. The observed precession amounted to a perihelion advance of approximately 565 seconds of arc per Earth century. Le Verriere then calculated an expected precession by considering the effects force each planet exerted on Mercury. This was an accurate, though computationally demanding technique. The outer planets, Le Verrier predicted, should cause an advance of 527 seconds of arc per century, leaving a residual 38 seconds that he was not able to explain using Newton’s theory (Baum, 136).[...]
An increase of approximately 10 percent in Venus’ mass would explain Mercury’s perihelion advance, but it would also affect Earth’s orbit in a way that had not been observed. Since the missing mass must not affect Earth, Le Verriere decided that it must be nearer to the Sun than Mercury’s orbit. And so began the hunt for Le Verriere’s ghost planet, or rather planets.”
—But this time, the planet never showed up. The answer wasn’t more stuff, but a completely new idea. Gravity had to be reimagined, and we invented general relativity.
#Pollock, Chris (2003): “Mercury’s Perihelion”
https://www.math.toronto.edu/~colliand/426_03/Papers03/C_Pollock.pdf
Quote: “In November 1915, Albert Einstein, sitting at a desk in Berlin, wrote his General Theory of Relativity. [...] Einstein did not set out to solve the problem of Mercury’s perihelion shift. In fact, the answer fell out as a neat consequence of his theory.
[...]
Einstein’s paper elegantly and convincingly explained the observed drift in perihelion without the need for ghost planets or asteroid belts. From Einstein’s original 1916 paper, entitled The Foundations of the General Theory of Relativity: ”Calculation gives for the planet Mercury a rotation of the orbit of 43” per century, corresponding exactly to astronomical observation (Le Verriere); for the astronomers have discovered in the motion of the perihelion of this planet, after allowing for all disturbances by other planets, an inexplicable remainder of this magnitude.” (Lorentz et al., 164)”
Einstein’s paper on the orbit of Mercury in translation:
#Vankov, Anatoli A. (2011): ”Einstein’s Paper: 'Explanation of the Perihelion Motion of Mercury from General Relativity Theory'”
https://etienneklein.fr/wp-content/uploads/2016/01/Relativit%C3%A9-g%C3%A9n%C3%A9rale.pdf
Original:
#Einstein, Albert (1915): “Erklärung der Perihelbewegung des Merkur aus der allgemeinen Relativitätstheorie”, Sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften, 831-839
https://ui.adsabs.harvard.edu/abs/1915SPAW.......831E/abstract
—The first signs that something deep could be off began piling up around 15 years ago – in the form of a few seemingly impossible cosmic monsters:
In this section we are considering ultra-large large-scale structures and the possible challenge they present to the cosmological principle. With “began pilling up”, we mean the moment that the rate at which we discovered this kind of structure increased. The existence of some of these structures is debated, as well as the exact scale where they could be considered to conflict with the cosmological principle. That means that there are many possible choices of the concrete moment when discoveries of ultra-large objects could be considered to “begin piling up”. We base our estimation on the following list of discoveries:
#Lopez, Alexia M.; Clowes, Roger G.; Williger, Gerard M. (2022): “A Giant Arc on the Sky”, Monthly Notices of the Royal Astronomical Society, vol. 516, 2, 1557–1572
Which shows an increase in the rate of discoveries starting around 2010-2012:
—A “giant arc” of galaxies over 3 billion light-years wide
The image on screen is an artistic depiction, and it is not to scale. More realistic depictions can be found in the following sources:
#University of Lancashire (2024): “A Big Cosmological Mystery” https://www.lancashire.ac.uk/news/big-ring-in-the-sky
#Lopez, Alexia M.; Clowes, Roger G.; Williger, Gerard M. (2022): “A Giant Arc on the Sky”, Monthly Notices of the Royal Astronomical Society, vol. 516, 2, 1557–1572
https://arxiv.org/abs/2201.06875
https://academic.oup.com/mnras/article/516/2/1557/6657809
—a massive group of quasars spanning 4 billion light-years
The image on screen is an artistic depiction. More realistic depictions can be found in the following sources:
#Clowes, Roger G. et al. (2013): ”A structure in the early Universe at z ∼ 1.3 that exceeds the homogeneity scale of the R-W concordance cosmology”, Monthly Notices of the Royal Astronomical Society, vol. 429, 4, 2910–2916
https://arxiv.org/abs/1211.6256
https://academic.oup.com/mnras/article/429/4/2910/1008743
Quote: “A large quasar group (LQG) of particularly large size and high membership has been identified in the DR7QSO catalogue of the Sloan Digital Sky Survey. It has characteristic size (volume1/3) ∼500 Mpc (proper size, present epoch), longest dimension ∼1240 Mpc, membership of 73 quasars and mean redshift z= 1.27. [...]
This new, HugeLQG appears to be the largest structure currently known in the early Universe. Its size suggests incompatibility with the Yadav et al. scale of homogeneity for the concordance cosmology, and thus challenges the assumption of the cosmological principle.”
With the following figure:
—a ring of galaxies 5 billion light-years across
The image on screen is an artistic depiction. More realistic depictions can be found in the following sources:
#Balázs, Lajos G. et al. (2015): “A giant ring-like structure at 0.78 < z < 0.86 displayed by GRBs”, Monthly Notices of the Royal Astronomical Society, vol. 452, 3, 2236–2246
https://academic.oup.com/mnras/article/452/3/2236/1078524
Quote: “We report here the discovery of the largest regular formation in the observable Universe; a ring with a diameter of 1720 Mpc, displayed by 9 gamma-ray bursts (GRBs), exceeding by a factor of 5 the transition scale to the homogeneous and isotropic distribution.”
#Bagoly, Zsolt et al (2022): ”The Spatial Distribution of Gamma-Ray Bursts with Measured Redshifts from 24 Years of Observation”, Universe, vol.8, 342
—and an unfathomable wall of galaxies stretching TEN billion light-years from end to end
The image on screen is an artistic depiction.
#Horváth, István et al. (2015): “New data support the existence of the Hercules-Corona Borealis Great Wall” Astronomy & Astrophysics, vol. 584, A48
https://www.aanda.org/articles/aa/full_html/2015/12/aa24829-14/aa24829-14.html
Quote: “The larger gamma-ray burst database further supports the existence of a statistically significant gamma-ray burst cluster at 1.6 ≤ z < 2.1 with an estimated angular size of 2000–3000 Mpc.”
—a whopping 10% of the entire observable universe.
One’s first estimation of the observable universe's size may be that it is 13.4 billion light-years, since we know the Big Bang happened 13.4 billion years ago. However, due to the expansion of space, galaxies that emitted photons in our direction eons ago are now much farther than they were when they emitted those photons, and so, it turns out that we can observe galaxies that are now much more than 13.4 billion light-years away.
#NASA’s Goddard Space Flight Center (2017): “Age & Size of the Universe Through the Years”(retrieved 2025)
https://imagine.gsfc.nasa.gov/educators/programs/cosmictimes/educators/guide/age_size.html
Quote: “The most distant objects in the Universe are 47 billion light years away, making the size of the observable Universe 94 billion light years across. How can the observable universe be larger than the time it takes light to travel over the age of the Universe? This is because the universe has been expanding during this time.”
#Horváth, István et al. (2015): “New data support the existence of the Hercules-Corona Borealis Great Wall” Astronomy & Astrophysics, vol. 584, A48
https://www.aanda.org/articles/aa/full_html/2015/12/aa24829-14/aa24829-14.html
Quote: “The larger gamma-ray burst database further supports the existence of a statistically significant gamma-ray burst cluster at 1.6 ≤ z < 2.1 with an estimated angular size of 2000–3000 Mpc.”
The largest estimate for the size of this structure, 3000 Mpc, is equivalent to 9.8 billion light-years.That represents approximately a 10% of the diameter of the observable universe.
—There are also monstrous voids: haunting cosmic deserts with far fewer galaxies than normal.
Cosmic voids are part of the large scale structure of the universe:
#Lemonick, Michael D. (2024): “How Analyzing Cosmic Nothing Might Explain Everything”, Scientific American
https://www.scientificamerican.com/article/how-analyzing-cosmic-nothing-might-explain-everything/
But some of them are so big and so empty that their existence defies the cosmological principle:
#Kopylov, Alexander I.; Kopylova, Flera G. (2002): “Search for streaming motion of galaxy clusters around the Giant Void”, Astronomy & Astrophysics, vol. 382, 2, 389 - 396
https://www.aanda.org/articles/aa/pdf/2002/05/aa1614.pdf
Quote: “One of the results of the programme “The Northern Cone of Metagalaxy”(Kopylov et al. 1988) was the discovery of the Giant Void (GV) (with a size of 400 Mpc and a redshift of the centre 0.116) in the distribution of very rich (R ≥ 2) Abell clusters”
—And according to some surveys, we happen to be living deep inside one of them – a gargantuan “local hole” 2 billion light-years across.
The exact size of the KBC void or “local hole” is hard to pin down. Estimates for its characteristic size range from 150 to 300 Mpc, often expressed as a “depth” of the hole measured from our point of view. Interpreting these “depths” as the approximate radius of the “hole” gives us a diameter of up to 600 Mpc, or 2 billion light-years.
#Keenan, Ryan C.; Barger, Amy J.; Cowie, Lennox L. (2013) : “Evidence for a ~300 Megaparsec Scale Under-density in the Local Galaxy Distribution”, The Astrophysical Journal, vol. 775, 1, 62
https://iopscience.iop.org/article/10.1088/0004-637X/775/1/62
#Frankel, Miriam (2024): “We live in a cosmic void so empty that it breaks the laws of cosmology”, New Scientist
Quote: “Mounting evidence suggests our galaxy sits at the centre of an expanse of nothingness 2 billion light years wide.”
—Well, the universe is organized in ever larger structures: galaxies, galaxy clusters, superclusters and eventually filaments – truly gargantuan structures separated by equally enormous voids.
#COSMOS - The SAO Encyclopedia of Astronomy: “Large-scale Structure” (retrieved 2024)
https://astronomy.swin.edu.au/cosmos/l/Large-scale+Structure
—But our cosmic theory says that these things can’t get arbitrarily large. At distances beyond one billion light-years or so, the filaments and voids should blur into a uniform soup.
“Our cosmic theory” refers to the ΛCDM model, considered the “standard model” of cosmology.
#Grayson, Skylar et al. (2025): "Guide to ΛCDM”, Astrobites
https://astrobites.org/2025/01/06/lambda_cdm/
Scientists use simulations based on the ΛCDM model to come up with precise predictions of how the universe should look like at large scales.The scales at which the simulated universes could be considered homogenous are around 370 Mpc or 1.2 billion light years.
#Aluri, Pavan K. (2023): “Is the observable Universe consistent with the cosmological principle?“, Classical and Quantum Gravity, vol. 40, 9, 094001
https://iopscience.iop.org/article/10.1088/1361-6382/acbefc/pdf
Quote: “Theoretically, within the flat ΛCDM model, N-body simulations have led to an upper bound on the homogeneity scale of 260 h−1 Mpc. With h ∼ 0.7, this scale equates to ∼370 Mpc.”
To match the simulated universes we create using the ΛCDM model, the real universe should be homogeneous at this scale.
Note that this number is relative. Smaller structures can also present a challenge to the idea of an homogeneous universe if they are strange enough, for example if they have a much higher or lower density than average for the universe as a whole.
—Our understanding of the universe is based on one key assumption – the cosmological principle. The idea that, if you zoom out far enough, the universe should be uniform, looking the same everywhere.
The cosmological principle states that the universe looks the same from every point and in all directions at sufficiently large scales.
#Aluri, Pavan K. (2023): “Is the observable Universe consistent with the cosmological principle?“, Classical and Quantum Gravity, vol. 40, 9, 094001
https://iopscience.iop.org/article/10.1088/1361-6382/acbefc/pdf
Quote: “The cosmological principle (CP) is a working assumption in modern cosmology that can be simply stated as the Universe is (statistically) isotropic and homogeneous at suitably large scales. This statement is admittedly vague, but nevertheless intuitive, and most importantly, practically very useful. Simply put, there exists a length scale beyond the reach of the rich structures that we observe in the local Universe, namely stars, galaxies and galaxy clusters, where the Universe should look the same in all directions. In other words, the Universe is isotropic. Moreover, this statement must hold true for all observers; if observer A erects a telescope somewhere else in the cosmos, she is expected to recover a Universe that looks the same as observer B’s Universe at an expected scale. Given enough observers seeing isotropic universes, it is once again intuitive that this guarantees there is no special place in the Universe, or alternatively that the Universe is homogeneous.”
—This is crucial because it means that our limited view of the cosmos is a fair sample of the whole. That, even if we are tiny creatures living in a speck of dust, we can learn things about the entire universe.
The cosmological principle allows us to extrapolate what we learn from the observable universe to the universe as a whole
#Beisbart, Claus (2010): “Can We Justifiably Assume the Cosmological Principle in Order to Break Model Underdetermination in Cosmology?”, Journal for General Philosophy of Science, vol. 40, 175–205
https://link.springer.com/article/10.1007/s10838-009-9098-9
Quote: “Conventional wisdom has it that cosmology is the science of the universe. ‘‘Cosmology.
The Science of the Universe’’ is in fact the title of a book written by a famous cosmologist
(Harrison 2000). As another author, Peacock (1999, xi) puts it,
cosmology [...] has the modest aim of understanding the entire universe and all its
content.
(cf. also Narlikar 1983, 1).
Not that such an ambitious view of cosmology is without alternatives. We may say, for instance, that cosmology deals with physics at the largest scales available to us or with the observable universe. [...]
As it happens, the attempts to obtain knowledge about the universe face a serious underdetermination problem [...]
In the last nine decades, cosmology has very much been shaped by the Cosmological Principle (CP, for short). According to that principle, the universe is spatially homogeneous and isotropic at large scales. The intuitive picture is that, at each particular time, different parts of space look more or less the same (spatial homogeneity). Furthermore, for any potential observer at any time, observations are not different for different directions (spatial isotropy around any observer at any time). Clearly, the principle can help us pick a unique model of the universe. If the universe is isotropic and homogeneous and if our observations provide a fair sample of the whole universe, then we can safely project the properties that we find in the observable universe to the whole universe.”
—Because if the universe isn’t the same everywhere,we could be like ants trying to guess the flavor of a cake while sitting on its only cherry. Everything we see might just be local weirdness – a cosmic quirk that doesn’t tell us the actual story of the universe.
The cosmological principle allows us to extrapolate what we learn from our observations to the universe as a whole.
#Beisbart, Claus (2010): “Can We Justifiably Assume the Cosmological Principle in Order to Break Model Underdetermination in Cosmology?”, Journal for General Philosophy of Science, vol. 40, 175–205
https://link.springer.com/article/10.1007/s10838-009-9098-9
But if the cosmological principle is not true, that would mean that different observers at different points in the universe could have very different pictures of it, even at the largest scales. This could lead them to completely different ideas about the laws of physics and cosmology, completely different stories about how the universe works.
—The next crack appeared about 10 years ago . It tore straight at the fabric of space — challenging how fast it grows.
In this chapter, we will discuss the Hubble tension, the difference in the values of the expansion rate of the universe —the Hubble constant— when measured by two different methods.
Many different values of the Hubble constant have been calculated since the discovery of the expansion of the universe, but up until the last couple of decades, scientists considered the differing values a result of the lack of precision in their measurements and hoped that more precise measurements would make all these results converge to a single consensus value.
However, increasingly precise measurements have yielded two different values. This discrepancy started attracting attention with the 2013 Planck results, which measured a lower value of the Hubble constant than was expected, despite an overall very good fit to the ΛCDM model.
#ESA (2013): “Planck reveals an almost perfect Universe” (retrieved 2025)
Quote: “Finally, the Planck data also set a new value for the rate at which the Universe is expanding today, known as the Hubble constant. At 67.15 kilometres per second per megaparsec, this is significantly less than the current standard value in astronomy.”
By 2015 the discrepancy was the subject of animated scientific discussion.
#Jackson, Neal (2015):”The Hubble Constant”, Living Reviews in Relativity, vol. 18, 2
https://link.springer.com/article/10.1007/lrr-2015-2
Quote: “Many, but not all, object-based measurements give H0 values of around 72–74 km s−1 Mpc−1, with typical errors of 2–3 km s−1 Mpc−1. This is in mild discrepancy with CMB-based measurements, in particular those from the Planck satellite, which give values of 67–68 km s−1 Mpc−1 and typical errors of 1–2 km s−1 Mpc−1. The size of the remaining systematics indicate that accuracy rather than precision is the remaining problem in a good determination of the Hubble constant. Whether a discrepancy exists, and whether new physics is needed to resolve it, depends on details of the systematics of the object-based methods, and also on the assumptions about other cosmological parameters and which datasets are combined in the case of the all-sky methods.”
—Every second the universe gets a little bigger.
#Encyclopedia Britannica: “Hubble constant”
https://www.britannica.com/science/Hubble-constant
—We know this because we have different ways to measure it and all confirm that space is expanding.
#Frank, Adam (2021): “The universe has a Hubble constant problem”, Big Think
Quote: “There are basically two modern ways to measure the Hubble constant. The first is based on looking at what cosmologists call the “late” universe. Astronomers try to make direct measurements of how fast distant objects are moving away from us (i.e., their redshift). There are two parts to these kinds of observations. First, astronomers need an accurate measurement of an object’s distance. Then they need to obtain an accurate measurement of its redshift. Using supernovae as “standard candles” for getting distances to far away galaxies, this late universe method gives a value of the Hubble constant of Ho = 74.03.
The other method relies on data from the “early” universe, i.e., right after the Big Bang. Microwave radiation emitted by matter about 300,000 years after the cosmic beginning provides astronomers with a rich source of early universe measurements. The best data from this cosmic microwave background comes from the Planck satellite launched back in 2009. And the best analysis of the Planck data yields Ho = 67.40, which is clearly not the same value as supernova data. Hence the two methods produce conflicting results. Not knowing which value is right, we can’t pin down other properties like, for example, the exact age of the universe.”
#Ding, Qianhang (2023): “A Novel Cosmological Probe and Model in Revealing Cosmic Tensions”, PhD Thesis, Hong Kong University of Science
Quote: “For a precise understanding in cosmic expansion history, various cosmological probes have been proposed in studying the cosmic evolution, mainly includes two categories of approaches, early epoch measurements and late epoch measurements.
Early epoch measurements:
Cosmological probes in early epoch measurements are using cosmic signals from the early universe to track the cosmic evolution, in which, CMB is a very important probe. According to angular power spectrum of CMB anisotropies, the sound horizon can be extracted, which gives a cosmological distance between the last scattering surface and us. Also, model parameters in LCDM cosmology can be determined, which provide a cosmic evolution at early epoch.
Late epoch measurements
Cosmological probes in late epoch measurements are using the late universe signals to construct distance-redshift relation, which is based on cosmic distance ladders. Its basic idea is that the cosmic distance can be measured step by step, where a small-scale distance is firstly measured and then works as a calibrator in a large-scale distance measurement. The cosmic distance ladder mainly uses several celestial objects or cosmic structure as distance indicators. For example, Cepheid variables and Type Ia supernovae (SNe) produce the predictable luminosity. The luminosity distance can be obtained by comparing their absolute and apparent magnitude, which ensures Cepheid variables and Type Ia SNe work as standard candles to provide luminosity distance-redshift relation. Baryon acoustic oscillations (BAO) determines a fixed sound horizon, by observing sound horizons at different redshifts. The calibration between angular diameter distance and redshift is extracted, which serves as standard rulers. Gravitational waves (GWs) from binary systems and their electromagnetic counterparts provide the calibration between the luminosity distance and redshift as standard sirens.”
—The problem? They can’t agree on how fast.
#Abdalla, Elcio et al. (2022): “Cosmology intertwined: A review of the particle physics, astrophysics, and cosmology associated with the cosmological tensions and anomalies”, Journal of High Energy Astrophysics, vol. 34, 49-211
https://www.sciencedirect.com/science/article/pii/S2214404822000179
https://arxiv.org/abs/2203.06142v1
Quote: “The most statistically significant and long-standing tension [in modern cosmology] is in the estimation of the Hubble constant H0 between the CMB data, that are cosmological model dependent and are obtained assuming a vanilla ΛCDM model, and the direct local distance ladder measurements.[...] In particular, we refer to the Hubble tension as the disagreement at 5.0σ between the Planck collaboration [24] value, H0 = (67.27 ± 0.60) km s−1 Mpc−1 at 68% confidence level (CL), and the latest 2021 SH0ES collaboration (R21 [44]) constraint, H0 = (73.04±1.04) km s−1 Mpc−1 at 68% CL, based on the Supernovae calibrated by Cepheids. However, there are not only these two values, but actually two sets of measurements, and all of the indirect model dependent estimates at early times agree between them, such as CMB and BAO experiments, and the same happens for all of the direct late time ΛCDM-independent measurements, such as distance ladders and strong lensing.”
This discrepancy has been re-affirmed by the latest observations with the James Webb Space Telescope
#Freedman, Wendy et al. (2025): “Status Report on the Chicago-Carnegie Hubble Program (CCHP): Measurement of the Hubble Constant Using the Hubble and James Webb Space Telescopes”, The Astrophysical Journal, vol. 985, 2
https://arxiv.org/pdf/2408.06153
Quote: “We present the latest results from the Chicago-Carnegie Hubble Program ( CCHP) to measure the Hubble constant, using data from the James Webb Space Telescope (JWST). The overall program aims to calibrate three independent methods: (1) Tip of the Red Giant Branch (TRGB) stars, (2) JAGB (J-Region Asymptotic Giant Branch) stars, and (3) Cepheids. To date, our program includes 10 nearby galaxies, hosting 11 Type Ia supernovae (SNe Ia) suitable for measuring the Hubble constant (H0). It also includes the galaxy NGC 4258, whose geometric distance provides the zero-point calibration. In this paper we discuss our results from the TRGB and JAGB methods. Our current best (highest precision) estimate is H0 = 70.39 ± 1.22 (stat) ± 1.33 (sys) ± 0.70 (σSN ), based on the TRGB method alone, with a total of 24 SN Ia calibrators from both HST and JWST data. Based on our new JWST data only, and tying into SNe Ia, we find values of H0 = 68.81 ± 1.79 (stat) ± 1.32 (sys) for the TRGB, and H0 = 67.80 ± 2.17 (stat) ± 1.64 (sys) km s−1 Mpc−1 for the JAGB method.[...] Indeed, if one retroactively corrects the R16 distances to the R22 scale correcting for the 0.035 mag offset, the SHoES H0 value becomes 72.05 ± 1.74 km s−1 Mpc−1”
#Kruesi, Liz (2024): ”The Webb Telescope Further Deepens the Biggest Controversy in Cosmology”, Quanta Magazine
—The important part is that, as measurements and calculations have become more and more precise, the disagreement has become only worse. By now, the chance that this mismatch is just an accidental fluke is less than one in a million.
#Riess, Adam G. et al. (2022): “A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km s−1 Mpc−1 Uncertainty from the Hubble Space Telescope and the SH0ES Team”, The Astrophysical Journal Letters, vol. 934, 1
https://iopscience.iop.org/article/10.3847/2041-8213/ac5c5b
https://arxiv.org/abs/2112.04510
Quote: “Our baseline result from the Cepheid-SN sample is H0=73.04+-1.04 km/s/Mpc, which includes systematics and lies near the median of all analysis variants. We demonstrate consistency with measures from HST of the TRGB between SN hosts and NGC 4258 with Cepheids and together these yield 72.53+-0.99. Including high-z SN Ia we find H0=73.30+-1.04 with q0=-0.51+-0.024. We find a 5-sigma difference with H0 predicted by Planck+LCDM, with no indication this arises from measurement errors or analysis variations considered to date. The source of this now long-standing discrepancy between direct and cosmological routes to determining the Hubble constant remains unknown. [...]
The full difference from the Planck+ΛCDM prediction of H0 = 67.4 ± 0.5 (Planck Collaboration et al. 2020) in units of Δ5 log H0 is 0.176 ±0.035 mag (errors in quadrature), a difference of 5.0σ (one in 3.5 million).”
#Poulin, Vivian (2025): “The Hubble tension”
https://cerncourier.com/a/the-hubble-tension/
Quote: “The great strength of the SH0ES programme is its use of NASA and ESA’s Hubble Space Telescope (HST, 1990–) at all three rungs of the distance ladder, bypassing the need for cross-calibration between instruments. SN Ia can be calibrated out to 40 Mpc. As a result, in 2022 SH0ES used measurements of 300 or so high-z SN Ia deep within the Hubble flow to measure H0 = 73.04 ± 1.04 km/s/Mpc. This is in more than 5σ tension with Planck’s ΛCDM prediction of 67.4 ± 0.5 km/s/Mpc.”
For an explanation on what these “σ”s mean and why scientists use them to talk about the significance of the disagreement between measurements check out this link:
#Chandler, David L.: “Explained: Sigma”, MIT News
https://news.mit.edu/2012/explained-sigma-0209
The graph on screen is taken from:
#Ezquiaga, Jose María; Zumalacarregui, Miguel (2018): “Dark Energy in Light of Multi-Messenger Gravitational-Wave Astronomy”, Frontiers in Astronomy and Space Sciences, vol. 5, 12
—The universe is literally giving us two different answers to the same question. So something fundamental must be broken – either our measurements of the universe, or our basic understanding of it.
The two different values of the Hubble constant come from local measurements on the one hand and from measurements from the early universe that rely on calculations with the ΛCDM model on the other. If all measurements are correct and we have interpreted them correctly, then the model, our “basic understanding of the universe”, must be wrong. However, if the ΛCDM model is correct, then there must be some mistake in our measurements or how we have interpreted them.
—The latest surprise is only 3 years old. It shattered a key part of our cosmic timeline – how and when the first galaxies formed.
In this chapter we review some of the surprising observations about early galaxies released from 2022 by the James Webb Space Telescope.
#Webb Space Telescope: “Webb Science: Galaxies Through Time” (retrieved 2025)
https://webbtelescope.org/contents/articles/webb-science-galaxies-through-time
—Telescopes act like time machines. Light from distant galaxies takes so long to reach us that we don’t see them as they are now, but as they were in the past.
#NASA: “Webb Science: Galaxies Through Time” (retrieved 2025)
#Webb Space Telescope: “How Does Webb See Back in Time?” (retrieved 2025)
https://webbtelescope.org/contents/articles/how-does-webb-see-back-in-time
—In 2021 we launched the James Webb, the most powerful space telescope ever built
#NASA: “James Webb Space Telescope” (retrieved 2025)
https://science.nasa.gov/mission/webb/
Quote: “Webb launched on Dec. 25th 2021. [...]
The Webb Space Telescope is the largest, most powerful and most complex telescope ever launched into space . Its design and development history stretches back before the Hubble Space Telescope was launched.”
#NASA: “Observatory: Hubble vs. Webb: On The Shoulders of a Giant” (retrieved 2025)
https://science.nasa.gov/mission/hubble/observatory/hubble-vs-webb/
Quote: “The James Webb Space Telescope is the largest, most technically advanced telescope ever built. Its larger size and richer infrared views allow it to go beyond Hubble’s Deep Field observations to peer back over 13.5 billion years, witnessing the first stars and galaxies forming out of the darkness of the early universe.”
—Almost immediately, it began finding massive galaxies so distant that they belong to a time when the universe was extremely young.
The James Webb Space Telescope started observing deep space and finding very bright and massive galaxies during its firsts observations, which took place immediately after the end of its commissioning, in July of 2022. For comparison, Hubble was launched in 1990 but could only image galaxies of comparable age in the Hubble Ultra-Deep Field, in 2003.
#NASA: “James Webb Space Telescope” (retrieved 2025)
https://science.nasa.gov/mission/webb/
Quote: “Webb's first science images released in July of 2022 just after commissioning was completed.”
#NASA: “Webb Images: First Images” (retrieved 2025) https://science.nasa.gov/mission/webb/multimedia/images/#First-Images
#Webb Space Telescope: “Webb's First Deep Field (NIRSpec MSA Emission Spectra)” (retrieved 2025)
https://webbtelescope.org/contents/media/images/2022/035/01G7HRYVGM1TKW556NVJ1BHPDZ
—The problem? Some are so premature that they date back to 280 million years after the Big Bang – far earlier than anyone expected.
The galaxy MoM-z14 has been spectroscopically confirmed to have a redshift of zspec = 14.44, which according to our current cosmological models would place it only 280 million years after the Big Bang.
#Naidu, Rohan P. et al. (2025): A Cosmic Miracle: A Remarkably Luminous Galaxy at zspec = 14.44 Confirmed with JWST, Preprint
https://arxiv.org/abs/2505.11263
Quote: “JWST has revealed a stunning population of bright galaxies at surprisingly early epochs, z > 10, where few such sources were expected. Here we present the most distant example of this class yet –, a luminous (MUV = −20.2) source in the COSMOS legacy field at zspec = 14.44+0.02−0.02 that expands the observational frontier to a mere 280 million years after the Big Bang.The redshift is confirmed with NIRSpec/prism spectroscopy through a sharp Lyman-α break and ≈ 3σ detections of five rest-UV emission lines.”
And there have been other spectroscopically confirmed bright galaxies at redshifts z~14:
#Carniani, Stefano (2024): “Spectroscopic confirmation of two luminous galaxies at a redshift of 14”, Nature, vol. 633, 318–322
https://www.nature.com/articles/s41586-024-07860-9
As well as other bright galaxies at unexpectedly high redshifts, though not all have been confirmed spectroscopically
#McGaugh, Stacy S. et al. (2024): “Accelerated Structure Formation: The Early Emergence of Massive Galaxies and Clusters of Galaxies”, The Astrophysical Journal, vol. 976, 1
Quote: “Figure 6. Mass estimates for high-redshift galaxies from JWST. Colored points based on photometric redshifts are from N. J. Adams et al. (2023; dark blue triangles), H. Atek et al. (2023; green circles), I. Labbé et al. (2023; open squares), R. P. Naidu et al. (2022; open star), Y. Harikane et al. (2023; yellow diamonds), C. M. Casey et al. (2024; light blue left-pointing triangles), and B. Robertson et al. (2024; orange right-pointing triangles). Black points from B. Wang et al. (2023; squares), S. Carniani et al. (2024; triangles), Y. Harikane et al. (2024; circles) and M. Castellano et al. (2024; star) have spectroscopic redshifts.[...]”
These are brighter than astronomers anticipated for such early galaxies:
#Webb Space Telescope (2024): “Webb Science: Galaxies Through Time”
https://webbtelescope.org/contents/articles/webb-science-galaxies-through-time
Quote: “In another intriguing find in the early universe, there are more bright galaxies than astronomers anticipated. The cause of this is still uncertain and needs to be investigated, but it is clear that galaxies were forming stars earlier, and more abundantly, than computer modeling suggested they would.”
Now we know that these galaxies are less massive than we thought when they were first discovered. However, there are still more bright and massive galaxies than we can fully account for.
#Webb Space Telescope (2024): “Webb Finds Early Galaxies Weren't Too Big for Their Britches After All”
https://webbtelescope.org/contents/news-releases/2024/news-2024-134
Quote: “There are still roughly twice as many massive galaxies in Webb’s data of the early universe than expected from the standard model. One possible reason might be that stars formed more quickly in the early universe than they do today.
“Maybe in the early universe, galaxies were better at turning gas into stars,” Chworowsky said.”
—Our theory says that the amorphous soup of matter that emerged from the Big Bang gave rise to the first galaxies through a long chain of mergers. Tiny lumps of dark and normal matter gathered under gravity, building larger chunks that then fused into even bigger ones, and so on and so on.
#Cho, Adrian (2018): “Galaxy simulations are at last matching reality—and producing surprising insights into cosmic evolution”, Science
Quote: “For decades, scientists have tried to simulate how the trillions of galaxies in the observable universe arose from clouds of gas after the big bang. But in the past few years, thanks to faster computers and better algorithms, the simulations have begun to produce results that accurately capture both the details of individual galaxies and their overall distribution of masses and shapes. [...] The universe sprang into existence in the big bang as a hot, dense soup of subatomic particles. Within a sliver of a second, it underwent an exponential growth spurt called inflation, which stretched infinitesimal quantum fluctuations in the particle soup into gargantuan ripples. Slowly, dense regions of dark matter coalesced under their own gravity into a vast tangle of clumps and filaments known as the cosmic web. Attracted by the dark matter's gravity, gas settled into the clumps, also called haloes, and condensed into the fusing balls of hydrogen called stars. By 500 million years after the big bang, the first galaxies had formed.”
#Genzel, Reinhard (2013): “The evolution of galaxies”, Max-Planck-Gesellschaft
https://www.mpg.de/7298320/the-evolution-of-galaxies
Quote: “From precision measurements of the cosmic microwave background and the large scale structure of galaxy distributions on the one hand, and very large computer simulations on the other, we can broadly understand the formation of galaxies in the ‘cold dark matter model’. [...] All numerical simulations of large scale structure evolution, such as the large “Millenium Simulation”carried out by Simon White and Volker Springel at MPA, find that large scale structure builds up hierarchically from smaller structures to larger sizes, from smaller mass to larger mass. From the galaxy’s perspective, this means that the embryonic galaxy grew over time from gas streams fed from the cosmic‘web’, including, from time to time, an incoming smaller galaxy/halo in that stream (a so called ‘minor merger’). More rarely (once every 3 years or so for a massive galaxy), there would also occur a highly dissipative ‘major merger’, at the end of which two disk galaxies were permanently transformed to a larger spheroidal galaxy.”
—But this process is lengthy. By our best estimates, the first large galaxies emerged 500 billion years after the Big Bang or so, not much before.
#Cho, Adrian (2018): “Galaxy simulations are at last matching reality—and producing surprising insights into cosmic evolution”, Science
Quote: “The universe sprang into existence in the big bang as a hot, dense soup of subatomic particles. Within a sliver of a second, it underwent an exponential growth spurt called inflation, which stretched infinitesimal quantum fluctuations in the particle soup into gargantuan ripples. Slowly, dense regions of dark matter coalesced under their own gravity into a vast tangle of clumps and filaments known as the cosmic web. Attracted by the dark matter's gravity, gas settled into the clumps, also called haloes, and condensed into the fusing balls of hydrogen called stars. By 500 million years after the big bang, the first galaxies had formed.”
More modern simulations are compatible with some of the early galaxies observations, though they still tend to underestimate the number and mass of galaxies at higher redshifts
#Keller, Benjamin W. (2023): ”Can Cosmological Simulations Reproduce the Spectroscopically Confirmed Galaxies Seen at z ≥ 10?”, The Astrophysical Journal Letters, vol. 943, 2
https://iopscience.iop.org/article/10.3847/2041-8213/acb148
Quote: “At higher redshift, only Simba and OBELISK produce galaxies as massive as those found in JADES. The number density of galaxies inferred from JADES is slightly larger than what is predicted by Simba at z = 11 and z = 12 but at a low level of significance. Overall, there appears to be no strong tension between models for galaxy formation in cosmological hydrodynamic simulations and the most distant spectroscopically confirmed galaxies.”
#McCaffrey, Joe et al. (2025): “Beyond No Tension: JWST z > 10 Galaxies Push Simulations to the Limit” (Preprint)
https://arxiv.org/abs/2509.07695
Quote: “We investigate in this letter whether these newly discovered galaxies are in conflict with the Renaissance simulations and thus whether they are causing tension with our established models of cosmology and/or high-redshift astrophysics. We discover that MoM-z14's high mass at early redshift can be explained by the Renaissance simulation suite, whereas the extremely high stellar mass of GS-z14 remains an outlier when compared to previous measurements of high-redshift galaxies detected by JWST and our numerical models (even after accounting for cosmic variance).”
The significance of this discrepancy will evolve as better observations and improved models arise.
—Matter in the baby universe was made up almost entirely of hydrogen and helium. Heavy elements like carbon or nitrogen were only forged in the cores of stars, which had to explode to release them.
#Harvard & Smithsonian Center for Astrophysics: “Early Universe” (retrieved 2025)
https://www.cfa.harvard.edu/research/topic/early-universe
Quote: “Researchers are using the best observatories in the world both to study the dark age and to find evidence for the first stars in the universe. As the first stars and black holes formed, they turned much of the hydrogen gas in the universe into plasma again, a process astronomers call “reionization”. The environment producing the earliest stars was radically different than star-forming regions today. The raw ingredients were almost exclusively hydrogen and helium, since stars themselves produce heavier elements through nuclear fusion.”
#CERN: “The early universe” (retrieved 2025)
https://home.cern/science/physics/early-universe
Quote: “In the first moments after the Big Bang, the universe was extremely hot and dense. As the universe cooled, conditions became just right to give rise to the building blocks of matter – the quarks and electrons of which we are all made. A few millionths of a second later, quarks aggregated to produce protons and neutrons. Within minutes, these protons and neutrons combined into nuclei. As the universe continued to expand and cool, things began to happen more slowly. It took 380,000 years for electrons to be trapped in orbits around nuclei, forming the first atoms. These were mainly helium and hydrogen, which are still by far the most abundant elements in the universe. Present observations suggest that the first stars formed from clouds of gas around 150–200 million years after the Big Bang. Heavier atoms such as carbon, oxygen and iron, have since been continuously produced in the hearts of stars and catapulted throughout the universe in spectacular stellar explosions called supernovae.”
—But some of these super-early galaxies contain heavy elements – meaning that entire generations of stars must have lived and died even before them.
#Webb Space Telescope (2024): “Webb Science: Galaxies Through Time”
https://webbtelescope.org/contents/articles/webb-science-galaxies-through-time
Quote: “In 2023, several teams of researchers published papers based on Webb data showing an abundance of chemicals, like nitrogen, in early galaxies. This presents a real puzzle, because nitrogen is produced by aged low-mass stars, and at this early phase of the universe, there has not been enough time for even one generation of low-mass stars to grow old and produce much nitrogen. And yet it is there.”
#ESO (2025): “Oxygen discovered in most distant known galaxy”
https://www.eso.org/public/news/eso2507/
Quote: “Researchers had thought that, at 300 million years old, the Universe was still too young to have galaxies ripe with heavy elements. However, the two ALMA studies indicate JADES-GS-z14-0 has about 10 times more heavy elements than expected.”
#Cameron, Alex J. et al. (2023) : “Nitrogen enhancements 440 Myr after the big bang: supersolar N/O, a tidal disruption event, or a dense stellar cluster in GN-z11?” Monthly Notices of the Royal Astronomical Society, vol. 523, 3
https://academic.oup.com/mnras/article/523/3/3516/7185828
Quote: “Recent observations of GN-z11 with JWST/NIRSpec revealed numerous oxygen, carbon, nitrogen, and helium emission lines at z = 10.6.”
#Naidu, Rohan P. et al. (2025): A Cosmic Miracle: A Remarkably Luminous Galaxy at zspec = 14.44 Confirmed with JWST, Preprint
https://arxiv.org/pdf/2505.11263
Quote: “The nitrogen emission and highly super-solar [N/C]> 1 hint at an abundance pattern similar to local globular clusters that may have once hosted luminous supermassive stars. ”
—Either the first galaxies sprouted in fast forward, or we are missing something huge about the infancy of the universe.
Multiple partial explanations have been proposed to explain the observations of early galaxies. Only more observations and further research will allow us to arrive at a conclusive answer.
It could be that we need to adjust our simulations to predict faster galaxy growth. Some more modern observations with higher definition yield more massive galaxies, though they still underestimate their number in comparison with current JWST observations.
#Keller, Benjamin W. (2023): ”Can Cosmological Simulations Reproduce the Spectroscopically Confirmed Galaxies Seen at z ≥ 10?”, The Astrophysical Journal Letters, vol. 943, 2
https://iopscience.iop.org/article/10.3847/2041-8213/acb148
Quote: “At higher redshift, only Simba and OBELISK produce galaxies as massive as those found in JADES. The number density of galaxies inferred from JADES is slightly larger than what is predicted by Simba at z = 11 and z = 12 but at a low level of significance. Overall, there appears to be no strong tension between models for galaxy formation in cosmological hydrodynamic simulations and the most distant spectroscopically confirmed galaxies.”
Alternatively there could be other unaccounted factors in our picture of the early evolution of the universe, from higher than expected or “bursty” star formation
#Airhart, Marc (2024): “Early Galaxies Were Not Too Big for Their Britches After All”
https://news.utexas.edu/2024/08/26/early-galaxies-were-not-too-big-for-their-britches-after-all/
#Sun, Guochao et al. (2023): “Bursty Star Formation Naturally Explains the Abundance of Bright Galaxies at Cosmic Dawn”, The Astrophysical Journal Letters, vol. 955, 2
https://iopscience.iop.org/article/10.3847/2041-8213/acf85a
Quote: “We have demonstrated that the FIRE-2 simulations with a multichannel implementation of standard stellar feedback processes can reproduce well the observed abundance of UV-bright galaxies at z ≳ 10. [...]. We further showed that the bursty SFH predicted to be common in galaxies at cosmic dawn is important for explaining the bright end of the UVLF.”
To even deeper physics
#Shen, Xuejian et al. (2024): “Early galaxies and early dark energy: a unified solution to the hubble tension and puzzles of massive bright galaxies revealed by JWST”, Monthly Notices of the Royal Astronomical Society, vol. 533, 4
https://academic.oup.com/mnras/article/533/4/3923/7750120
Quote: “We use an empirical galaxy formation model to explore the potential of alleviating these tensions through an Early Dark Energy (EDE) model, originally proposed to solve the Hubble tension. [...] [T]the implausibly large cosmic stellar mass densities inferred from some JWST observations are no longer in tension with cosmology when the EDE is considered.”
—Our theory also says that the Big Bang should have created 3 times more lithium than what we find in stars – a decades-old itch that cosmologists just can’t scratch.
#Bertulani, Carlos A.; Mukhamedzhanov, Akram M.; Shubhchintak (2016): “The cosmological lithium problem revisited”, AIP Conference Proceedings, 1753 (1)
https://arxiv.org/abs/1603.03864
https://pubs.aip.org/aip/acp/article-abstract/1753/1/040001/791172/The-cosmological-lithium-problem-revisited
Quote: “Big Bang nucleosynthesis leads to robust predictions which have survived the test of observations. Observations of lithium abundance in metal poor halo stars [12] yield 7Li/H = 1.58+0.35−0.28 × 10−10 which is appreciably smaller than the value of 7Li/H = 4.46 × 10−10 predicted by Big Bang Nucleosynthesis (BBN) [13]. This difference has survived
a large number of tests and is the source of the lithium puzzle. ”
#Fields, Brian D. (2011): “The Primordial Lithium Problem”, Annual Review of Nuclear and Particle Science, 61, 47-68
https://arxiv.org/pdf/1203.3551
Quote: “Big-bang nucleosynthesis (BBN) theory, together with the precise WMAP cosmic baryon density, makes tight predictions for the abundances of the lightest elements. Deuterium and 4He measurements agree well with expectations, but 7Li observations lie a factor 3 − 4 below the BBN+WMAP prediction. This 4 − 5σ mismatch constitutes the cosmic “lithium problem,” with disparate solutions possible.”
This problem persists after the latest analysis of the Planck results:
#Fields, Brian D. et al. (2020): “Big-Bang Nucleosynthesis after Planck”, Journal of Cosmology and Astroparticle Physics, vol. 2020, 3
https://iopscience.iop.org/article/10.1088/1475-7516/2020/03/010
Quote: “Most importantly the lithium problem remains, and indeed is more acute given the very tight D/H observational constraints; new neutron capture data reveals systematics that somewhat increases uncertainty and thus slightly reduces but does not essentially change the problem.”
—It predicts that dark matter should pile up sharply at galaxy centers, but instead we find gentle hills.
#Bullock, James S.; Boylan-Kolchin, Michael (2017): “Small-Scale Challenges to the ΛCDM Paradigm”, Annual Review of Astronomy and Astrophysics, 55, 343-387 https://www.annualreviews.org/content/journals/10.1146/annurev-astro-091916-055313
Quote: “The dark energy plus cold dark matter (ΛCDM) cosmological model has been a demonstrably successful framework for predicting and explaining the large-scale structure of the Universe and its evolution with time. Yet on length scales smaller than ∼1 Mpc and mass scales smaller than ∼1011M⊙, the theory faces a number of challenges. For example, the observed cores of many dark matter–dominated galaxies are both less dense and less cuspy than naïvely predicted in ΛCDM.”
—It says dark energy, the mysterious force pushing the universe apart, has stayed constant since the Big Bang. But last year, one of the biggest galaxy surveys ever conducted dropped the bombshell that it may have been changing over time.
The first DESI results that showed a preference for evolving dark matter came up in 2024, but were officially published 2025
#DESI Collaboration (2025): “DESI 2024 VII: cosmological constraints from the full-shape modeling of clustering measurements”, Journal of Cosmology and Astroparticle Physics, vol. 2025, 7
https://arxiv.org/abs/2411.12022
https://iopscience.iop.org/article/10.1088/1475-7516/2025/07/028
#DESI: “DESI 2024 Results: November 19 Guide” (retrieved 2025)
https://www.desi.lbl.gov/2024/11/19/desi-y1-results-nov-19-guide/
Quote: “Analysis of the first year of DESI data, in combination with other probes, shows preference for a cosmological model where the dark energy density evolves in time. This corresponds to the preference of the parameter w0 in the accompanying plot being different from -1, and wa different from 0. This result persists when we go beyond our earlier analysis of baryon acoustic oscillations signature in the clustering of galaxies, quasars, and features in quasar spectra, and extend it to utilize the full clustering signal of these cosmological tracers.[...]”
This figure shows constraints on w0 and wa in the dark energy equation of state parameterization w=w0+wa(1-a). The solid contours represent constraints based on the new DESI (BAO+FS) analysis combined with CMB and PantheonPlus, while the dashed contours represent the previous BAO-only DESI data and the same combinations with external datasets.”
Since then, this preference has strengthened with newer results:
#DESI Collaboration: “DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints”, Preprint
https://arxiv.org/abs/2503.14738
Quote: “The results are well described by a flat ΛCDM model, but the parameters preferred by BAO are in mild, 2.3σ tension with those determined from the cosmic microwave background (CMB), although the DESI results are consistent with the acoustic angular scale 𝜃* that is well-measured by Planck. This tension is alleviated by dark energy with a time-evolving equation of state parametrized by w0 and wa, which provides a better fit to the data [...]. This solution is preferred over ΛCDM at 3.1σ for the combination of DESI BAO and CMB data. When also including SNe, the preference for a dynamical dark energy model over CDM ranges from 2.8–4.2σ depending on which SNe sample is used.[...]Unless there is an unknown systematic error associated with one or more datasets, it is clear that CDM is being challenged by the combination of DESI BAO with other measurements and that dynamical dark energy offers a possible solution.”
#Biron, Lauren (2025): “New DESI Results Strengthen Hints That Dark Energy May Evolve”, News from Berkley Lab
https://newscenter.lbl.gov/2025/03/19/new-desi-results-strengthen-hints-that-dark-energy-may-evolve/
Quote: “Taken alone, DESI’s data are consistent with our standard model of the universe: Lambda CDM (where CDM is cold dark matter and Lambda represents the simplest case of dark energy, where it acts as a cosmological constant). However, when paired with other measurements, there are mounting indications that the impact of dark energy may be weakening over time and that other models may be a better fit. Those other measurements include the light leftover from the dawn of the universe (the cosmic microwave background or CMB), exploding stars (supernovae), and how light from distant galaxies is warped by gravity (weak lensing).
“We’re guided by Occam’s razor, and the simplest explanation for what we see is shifting,” said Will Percival, co-spokesperson for DESI and a professor at the University of Waterloo. “It’s looking more and more like we may need to modify our standard model of cosmology to make these different datasets make sense together — and evolving dark energy seems promising.”[...]
—If true, this would overturn our current picture of the universe, its past and its future.
Our understanding of the past and the future of the universe, including its age and its ultimate end, are based on the idea that the cosmological constant lambda is constant. If lambda evolves, there are different possible ends of the universe, which we addressed in this video.
#Kurzgesagt – In a Nutshell (2023): “How to Destroy the Universe”
—Even things that we considered settled beyond any doubt, like the interpretation of the cosmic microwave background, are suddenly up for debate – those early galaxies might have been bright enough to contaminate the signal.
#Gjergo, Eda ; Kroupa, Pavel (2025): “The impact of early massive galaxy formation on the cosmic microwave background”, Nuclear Physics B, 1017, 116931
https://www.sciencedirect.com/science/article/pii/S0550321325001403
Quote: “The Cosmic Microwave Background (CMB) anisotropies, corrected for foreground effects, form the foundation of cosmology and support the Big Bang model. A previously overlooked foreground component is the formation of massive early-type galaxies (ETGs), which can no longer be ignored, particularly in light of JWST's detection of massive, evolved systems at extreme redshifts (z>13).[...] The massive ETG evolution presented in this work is consistent with recent advancements in stellar and galaxy evolution, and is derived entirely without priors or constraints from the CMB. Yet, it emerges as a non-negligible source of CMB foreground contamination. Even in our most conservative estimates, massive ETGs account for 1.4% up to the full present-day CMB energy density.”
—Some scientists argue that these aren’t real cracks, but mirages that will disappear or raw gems that will end up sharpening our theories. Others are more radical and defend that we need completely new ideas.
Each of the “cracks” we have mentioned in this video is the subject of lively scientific discussion and worthy of consideration: But scientists are split on their significance and the scale of the changes to our models that they require.
Some claim that the “cosmic monsters” are compatible with the ΛCDM model
#Sawala, Till et al. (2025): “The Emperor's New Arc: gigaparsec patterns abound in a ΛCDM universe”, Preprint
https://arxiv.org/abs/2502.03515
while others disagree.
#Lopez, Alexia M.; Clowes, Roger G. (2025): “Gigaparsec structures are nowhere to be seen in ΛCDM: an enhanced analysis of LSS in FLAMINGO-10K simulations” , Preprint
https://arxiv.org/abs/2504.14940
Some consider that the the differing values of the expansion rate of the universe may be the result of a systematic error in local measurements and look for alternative ways of measuring the Hubble constant
#Lee, Abigail J. et al. (2025): “The Chicago-Carnegie Hubble Program: The JWST J-region Asymptotic Giant Branch (JAGB) Extragalactic Distance Scale”, The American Astronomical Society, 985, 2
https://iopscience.iop.org/article/10.3847/1538-4357/adc8a1
https://arxiv.org/abs/2408.03474
while others consider that it reveals real physics and could not be caused by systematic errors.
#Riess, Adam G. et al. (2024): “JWST Observations Reject Unrecognized Crowding of Cepheid Photometry as an Explanation for the Hubble Tension at 8σ Confidence” , The American Astronomical Society, 962, 1
https://iopscience.iop.org/article/10.3847/2041-8213/ad1ddd
https://arxiv.org/abs/2401.04773
There have been many explanations for JWST’s observations of early galaxies that explain them, at least partially, without the need to introduce changes to the ΛCDM model.
Some consider that the observations agree with our current simulations when we use higher resolutions
#McCaffrey, Joe et al. (2023): “No Tension: JWST Galaxies at z>10 Consistent with Cosmological Simulations”, The Open Journal of Astrophysics, 6
https://arxiv.org/abs/2304.13755
https://astro.theoj.org/article/88302-no-tension-jwst-galaxies-at-z-10-consistent-with-cosmological-simulations
or that there are biases in how we are interpreting the data.
#Ziegler, Joshua J. et al. (2025): “Explaining the "too massive" high-redshift galaxies in JWST data: numerical study of three effects and a simple relation”, Preprint
https://www.arxiv.org/abs/2507.21409
Many propose exciting changes in areas of physics like our notions of star or black hole formation in the early universe.
#Boyle, Rebecca (2024): “The ‘Beautiful Confusion’ of the First Billion Years Comes Into View”, Quanta Magazine
While others look for explanations beyond the ΛCDM model
#McGaugh et al. (2024): “Accelerated Structure Formation: The Early Emergence of Massive Galaxies and Clusters of Galaxies”, The Astrophysical Journal, vol 976, 1
https://iopscience.iop.org/article/10.3847/1538-4357/ad834d
—But whatever the case, the big picture is difficult to ignore – the sense of crisis is growing.
#Abdalla, Elcio et al. (2022): “Cosmology intertwined: A review of the particle physics, astrophysics, and cosmology associated with the cosmological tensions and anomalies”, Journal of High Energy Astrophysics, vol. 34, 49-211
https://arxiv.org/abs/2203.06142v1
Quote: “In view of the accumulation [of] the ΛCDM tensions discussed in the previous sections it is becoming apparent that the need for a new standard cosmological model has been increasing during the last few years.”
The authors further compare the present to the time just before the accelerated expansion of the universe was discovered.
Quote: “The present time has similarities with the late nineties when it was becoming apparent that the standard model of that time (flat sCDM based on the Einstein-de Sitter model) had been accumulating a range of inconsistencies with data”
This sentiment has echoed in popular science coverage:
#Ananthaswamy, Anil; Billings, Lee (2025): “The Hubble Tension Is Becoming a Hubble Crisis”, Scientific American
https://www.scientificamerican.com/article/the-hubble-tension-is-becoming-a-hubble-crisis/
—Science doesn’t move in a straight line, but in cycles – periods of calm followed by sudden crises. When a crisis hits, experiments start giving results that don’t fit existing theories, confusion grows, and strange ideas pop up. And eventually, there is a revolution. A deeper truth emerges, and a new cycle starts over again.
#Stanford Encyclopedia of Philosophy (2018): “Thomas Kuhn: The Development of Science”
https://plato.stanford.edu/entries/thomas-kuhn/#DeveScie
Quote: “According to Kuhn the development of a science is not uniform but has alternating ‘normal’ and ‘revolutionary’ (or ‘extraordinary’) phases. The revolutionary phases are not merely periods of accelerated progress, but differ qualitatively from normal science. Normal science does resemble the standard cumulative picture of scientific progress, on the surface at least. Kuhn describes normal science as ‘puzzle-solving’ (1962/1970a, 35–42). While this term suggests that normal science is not dramatic, its main purpose is to convey the idea that like someone doing a crossword puzzle or a chess problem or a jigsaw, the puzzle-solver expects to have a reasonable chance of solving the puzzle, that his doing so will depend mainly on his own ability, and that the puzzle itself and its methods of solution will have a high degree of familiarity. A puzzle-solver is not entering completely uncharted territory. Because its puzzles and their solutions are familiar and relatively straightforward, normal science can expect to accumulate a growing stock of puzzle-solutions.
[...]
It is only the accumulation of particularly troublesome anomalies that poses a serious problem for the existing disciplinary matrix. A particularly troublesome anomaly is one that undermines the practice of normal science. For example, an anomaly might reveal inadequacies in some commonly used piece of equipment, perhaps by casting doubt on the underlying theory. If much of normal science relies upon this piece of equipment, normal science will find it difficult to continue with confidence until this anomaly is addressed. A widespread failure in such confidence Kuhn calls a ‘crisis’ (1962/1970a, 66–76).
The most interesting response to crisis will be the search for a revised disciplinary matrix, a revision that will allow for the elimination of at least the most pressing anomalies and optimally the solution of many outstanding, unsolved puzzles. Such a revision will be a scientific revolution.”
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